[MLIR][SCF] Define -scf-rotate-while pass (llvm#99850)

Define SCF dialect patterns rotating `scf.while` loops leveraging
existing `mlir::scf::wrapWhileLoopInZeroTripCheck`. `forceCreateCheck`
is always `false` as the pattern would lead to an infinite recursion
otherwise.

This pattern rotates `scf.while` ops, mutating them from "while" loops to
"do-while" loops. A guard checking the condition for the first iteration
is inserted. Note this guard can be optimized away if the compiler can
prove the loop will be executed at least once.

Using this pattern, the following while loop:

```mlir
scf.while (%arg0 = %init) : (i32) -> i64 {
  %val = .., %arg0 : i64
  %cond = arith.cmpi .., %arg0 : i32
  scf.condition(%cond) %val : i64
} do {
^bb0(%arg1: i64):
  %next = .., %arg1 : i32
  scf.yield %next : i32
}
```

Can be transformed into:

``` mlir
%pre_val = .., %init : i64
%pre_cond = arith.cmpi .., %init : i32
scf.if %pre_cond -> i64 {
  %res = scf.while (%arg1 = %va0) : (i64) -> i64 {
    // Original after block
    %next = .., %arg1 : i32
    // Original before block
    %val = .., %next : i64
    %cond = arith.cmpi .., %next : i32
    scf.condition(%cond) %val : i64
  } do {
  ^bb0(%arg2: i64):
    %scf.yield %arg2 : i32
  }
  scf.yield %res : i64
} else {
  scf.yield %pre_val : i64
}
```

The test pass for `wrapWhileLoopInZeroTripCheck` has been modified to
use the new pattern when `forceCreateCheck=false`.

---------

Signed-off-by: Victor Perez <[email protected]>

Author: victor-eds

mlir/include/mlir/Dialect/SCF/Transforms/Patterns.h

@@ -85,6 +85,9 @@ void populateSCFForLoopCanonicalizationPatterns(RewritePatternSet &patterns);
 ///  * `after` block containing arith.addi
 void populateUpliftWhileToForPatterns(RewritePatternSet &patterns);
 
+/// Populate patterns to rotate `scf.while` ops, constructing `do-while` loops
+/// from `while` loops.
+void populateSCFRotateWhileLoopPatterns(RewritePatternSet &patterns);
 } // namespace scf
 } // namespace mlir
 

mlir/include/mlir/Dialect/SCF/Transforms/Transforms.h

@@ -228,6 +228,11 @@ FailureOr<ForOp> pipelineForLoop(RewriterBase &rewriter, ForOp forOp,
 ///   } else {
 ///     scf.yield %pre_val : i64
 ///   }
+///
+/// Failure mechanism is not implemented for this function, so it currently
+/// always returns a `WhileOp` operation: a new one if the transformation took
+/// place or the input `whileOp` if the loop was already in a `do-while` form
+/// and `forceCreateCheck` is `false`.
 FailureOr<WhileOp> wrapWhileLoopInZeroTripCheck(WhileOp whileOp,
                                                 RewriterBase &rewriter,
                                                 bool forceCreateCheck = false);

mlir/lib/Dialect/SCF/Transforms/CMakeLists.txt

@@ -13,6 +13,7 @@ add_mlir_dialect_library(MLIRSCFTransforms
   ParallelLoopCollapsing.cpp
   ParallelLoopFusion.cpp
   ParallelLoopTiling.cpp
+  RotateWhileLoop.cpp
   StructuralTypeConversions.cpp
   TileUsingInterface.cpp
   WrapInZeroTripCheck.cpp

mlir/lib/Dialect/SCF/Transforms/RotateWhileLoop.cpp

@@ -0,0 +1,44 @@
+//===- RotateWhileLoop.cpp - scf.while loop rotation ----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Rotates `scf.while` loops.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/SCF/Transforms/Patterns.h"
+
+#include "mlir/Dialect/SCF/IR/SCF.h"
+
+using namespace mlir;
+
+namespace {
+struct RotateWhileLoopPattern : OpRewritePattern<scf::WhileOp> {
+  using OpRewritePattern<scf::WhileOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(scf::WhileOp whileOp,
+                                PatternRewriter &rewriter) const final {
+    // Setting this option would lead to infinite recursion on a greedy driver
+    // as 'do-while' loops wouldn't be skipped.
+    constexpr bool forceCreateCheck = false;
+    FailureOr<scf::WhileOp> result =
+        scf::wrapWhileLoopInZeroTripCheck(whileOp, rewriter, forceCreateCheck);
+    // scf::wrapWhileLoopInZeroTripCheck hasn't yet implemented a failure
+    // mechanism. 'do-while' loops are simply returned unmodified. In order to
+    // stop recursion, we check input and output operations differ.
+    return success(succeeded(result) && *result != whileOp);
+  }
+};
+} // namespace
+
+namespace mlir {
+namespace scf {
+void populateSCFRotateWhileLoopPatterns(RewritePatternSet &patterns) {
+  patterns.add<RotateWhileLoopPattern>(patterns.getContext());
+}
+} // namespace scf
+} // namespace mlir

mlir/test/Dialect/SCF/wrap-while-loop-in-zero-trip-check.mlir

@@ -20,7 +20,7 @@ func.func @wrap_while_loop_in_zero_trip_check(%bound : i32) -> i32 {
 // CHECK-SAME:      %[[BOUND:.*]]: i32) -> i32 {
 // CHECK-DAG:     %[[C0:.*]] = arith.constant 0 : i32
 // CHECK-DAG:     %[[C5:.*]] = arith.constant 5 : i32
-// CHECK-DAG:     %[[PRE_COND:.*]] = arith.cmpi slt, %[[C0]], %[[BOUND]] : i32
+// CHECK-DAG:     %[[PRE_COND:.*]] = arith.cmpi sgt, %[[BOUND]], %[[C0]] : i32
 // CHECK-DAG:     %[[PRE_INV:.*]] = arith.addi %[[BOUND]], %[[C5]] : i32
 // CHECK:         %[[IF:.*]]:2 = scf.if %[[PRE_COND]] -> (i32, i32) {
 // CHECK:           %[[WHILE:.*]]:2 = scf.while (

mlir/test/lib/Dialect/SCF/TestSCFWrapInZeroTripCheck.cpp

@@ -1,4 +1,4 @@
-//===- TestWrapInZeroTripCheck.cpp -- Passes to test SCF zero-trip-check --===//
+//===- TestSCFWrapInZeroTripCheck.cpp -- Pass to test SCF zero-trip-check -===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -13,9 +13,11 @@
 
 #include "mlir/Dialect/Func/IR/FuncOps.h"
 #include "mlir/Dialect/SCF/IR/SCF.h"
+#include "mlir/Dialect/SCF/Transforms/Patterns.h"
 #include "mlir/Dialect/SCF/Transforms/Transforms.h"
 #include "mlir/IR/PatternMatch.h"
 #include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
 
 using namespace mlir;
 
@@ -46,13 +48,19 @@ struct TestWrapWhileLoopInZeroTripCheckPass
     func::FuncOp func = getOperation();
     MLIRContext *context = &getContext();
     IRRewriter rewriter(context);
-    func.walk([&](scf::WhileOp op) {
-      FailureOr<scf::WhileOp> result =
-          scf::wrapWhileLoopInZeroTripCheck(op, rewriter, forceCreateCheck);
-      // Ignore not implemented failure in tests. The expected output should
-      // catch problems (e.g. transformation doesn't happen).
-      (void)result;
-    });
+    if (forceCreateCheck) {
+      func.walk([&](scf::WhileOp op) {
+        FailureOr<scf::WhileOp> result =
+            scf::wrapWhileLoopInZeroTripCheck(op, rewriter, forceCreateCheck);
+        // Ignore not implemented failure in tests. The expected output should
+        // catch problems (e.g. transformation doesn't happen).
+        (void)result;
+      });
+    } else {
+      RewritePatternSet patterns(context);
+      scf::populateSCFRotateWhileLoopPatterns(patterns);
+      (void)applyPatternsAndFoldGreedily(func, std::move(patterns));
+    }
   }
 
   Option<bool> forceCreateCheck{